IMO double boxed is the way to go! I'm not a fan of putting eggs IN the egg cartons. IMO that adds more pressure to the eggs when the seller forces that carton closed. I received a box once that was positively bulging they crammed so much shredded paper in it. In the middle was the egg carton, forced closed, with eggs bubble wrapped inside. Half the eggs were crushed. Ironically I got a 100% hatch from the surviving eggs.
I've seen lots of methods of packing. IMO you got to pack so that the eggs cannot shift around inside the box. I compare that box to a car about to hit a brick wall. I make sure all my little eggies are wearing their seat belts!